Looking for a way to create the character of Patrick Bateman, Christian Bale stumbled onto a Tom Cruise appearance on the Late Show with David Letterman (1993). According to co-writer and director Mary Harron, Bale saw in Cruise "this very intense friendliness with nothing behind the eyes," and Bale subsequently based the character of Bateman on that. Interestingly, Tom Cruise is actually featured in the novel. He lives in the same apartment complex as Bateman, who meets him in an elevator and gets the name of Cocktail (1988) wrong, calling it "Bartender."
The single biggest cost on the film was purchasing the rights to the various songs used throughout.
During the shooting of the film, Christian Bale spoke in an American accent off-set at all times. At the wrap party, when he began to speak in his native British accent, many of the crew thought he was speaking that way as an accent for another film. They had thought he was American throughout the entire shoot.
During production, Christian Bale followed the morning routine that his character Patrick Bateman describes toward the beginning of the film.
The film had various problems with designer labels during production. Cerruti agreed to allow Christian Bale to wear their clothes, but not when the character was killing anyone. Rolex agreed that anyone in the film could wear their watches except Bateman (hence the famous line from the book "Don't touch the Rolex" had to be changed to "Don't touch the watch"). Perry Ellis provided underwear at the last minute after Calvin Klein pulled out of the project. Comme des Garçons refused to allow one of their overnight bags to be used to carry a corpse, so Jean Paul Gaultier was used instead.
Christian Bale: [weight transformation] This is the first film in which Christian Bale started his run on different physiques and body transformations. Others include Reign of Fire (2002), The Machinist (2004), Batman Begins (2005), Rescue Dawn (2006), The Dark Knight (2008), The Fighter (2010), The Dark Knight Rises (2012), American Hustle (2013), Vice (2018), and Ford v Ferrari (2019).